This invention relates to achieving both superior bending fatigue strength and pitting fatigue life of gear(s) and/or shaft components, using “conventional alloy steel” by a method having following steps in sequence.
Step 1: modified carbonitriding treatment, and
Step 2: hard shot peening process.
Carburizing, hardening and tempering (hereafter called only “carburized”) have been followed commonly over years for gear train transmission components in many designs so as to increase load carrying capacity. However, load carrying capability produced after carburizing is limited by microstructural and/or sub microstructural anomalies such as grain boundary oxidation, segregated carbides, bainite and alike anomalies. It has not been possible to extend, beyond certain limits, the load carrying capability of such transmissions without geometrical changes of components. Such geometrical changes in transmissions come with the following significant disadvantages: increases in weight, fuel consumption, development cost, development time and product cost and which ultimately results in increased customer dissatisfaction.
Geometrical changes in transmission components result in weight increase as mentioned above, and impose more loads on engines. Higher engine loads lead to higher emissions. To address higher emission problems, engine designs are required to undergo associated changes to reduce such emissions and this further increases the design and manufacturing costs.
Often space constraints in existing transmissions will make such geometric design changes very difficult to accommodate.
Several other surface treatment related techniques have evolved and been used in recent years to make surfaces and sub-surfaces more durable and reliable for higher torque transmitting capabilities of transmissions, already in use.
Some of the techniques available take advantage of the residual compressive stresses. However, such techniques have limited applications as they make use of special steels and/or elaborate heat treatment processes leading to higher production costs. Further, they are not able to produce simultaneous improvements in bending fatigue strength and pitting fatigue life.
Patent References:
1) U.S. Pat. No. 6,447,619 uses special steels with 0.3 to 3.0 weight % Aluminum and 0.2 to 2.0 weight % Vanadium. The disclosure claims increase in pitting life only and does not address bending fatigue strength, essential for gear(s) and/or shaft of the components. Further the special steel used for processing requires a special steel making process which increases production costs.
2) U.S. Pat. No. 5,595,613 claims to produce superior pitting resistance and wear resistance only with special steels having 1.5 to 5.0 weight % Chromium. The treatment does not address bending fatigue strength. Further, the special steel used for processing requires a special steel making process which increases production costs.
3) U.S. Pat. No. 5,019,182 claims to use a heat treatment route which does not address the tempering process. In the absence of the tempering process after quenching, quenching stresses are not relieved prior to service leading to dimensional instability and susceptibility to cracking. Further, the bending fatigue strength is not addressed in the claim.
In light of the existing prior-art, there has been a long standing demand to provide both superior bending and pitting fatigue life on gear(s) and/or shaft components simultaneously using “conventional alloy steel” which is described as the cheaper, most widely used and widely available steels for gear(s) and/or shaft components. None of the above disclosures provide complete solutions for producing both superior bending fatigue strength and pitting fatigue life simultaneously.